Republicans’ plan to repeal and replace Obamacare would cut spending by more than $300 billion over the next decade but would result in 24 million fewer people holding health insurance, Congress’ official scorekeeper said Monday, creating another political hurdle for President Trump and Republican leaders on Capitol Hill.

The losses would build over time, with 14 million forgoing coverage next year — nearly half of them the younger, healthier people who would no longer be forced to buy plans under Obamacare’s individual mandate.

Over time, bigger losses would come from the ranks of Medicaid, where the Republican bill rolls back the program’s expansion and challenges states to experiment, trying to cover more people with less money.

By 2026, some 14 million fewer people would be covered by Medicaid, while 10 million others would either refuse to buy plans, be kicked off their employers’ insurance or otherwise be knocked out of coverage.

All told, the CBO estimated that 52 million people would be uninsured by the end of the budget window, compared with about 28 million who would lack insurance under the current system.

“How can they look their constituents in the eye and say 24 million of you will no longer have coverage?” said House Minority Leader Nancy Pelosi, California Democrat and a driving force behind the 2010 Affordable Care Act that Republicans are working to repeal.

Republicans were divided over their response to the CBO.

House Speaker Paul D. Ryan, Wisconsin Republican, called the numbers encouraging. He said the $337 billion in savings projected by the CBO gives lawmakers more room to tweak the bill.

But the Trump administration expressed doubt that people on Medicaid would give up coverage they already have.

“We believe that our plan will cover more individuals at a lower cost and give them the choices that they want for the coverage that they want for themselves and their family, not that the government forces them to buy,” Health and Human Services Secretary Tom Price said.

He said the CBO score doesn’t take into account executive actions and additional Republican legislation. Neither of those has been detailed, so it’s unclear what effect they would have.

Still, Mr. Price and White House Budget Director Mick Mulvaney said there was some good news in the CBO score: It predicts premiums will fall by 10 percent after a decade, meaning consumers will save money upfront when they no longer have to pay for types of coverage they don’t need, but which was mandated by Obamacare.

The Republican plan curtails the expansion of Medicaid, the health care program for the poor, and turns it into a block grant, sending a defined amount of money to states to experiment with coverage.

Combined, those are expected to save the government about $1.2 trillion over the next decade.

Republicans also repeal the health law’s taxes on insurers, high earners and others, costing the Treasury Department nearly $900 billion in projected revenue.

The CBO score loomed over the health care debate for weeks, as Mr. Trump and House GOP leaders try to thread the needle between conservatives — who want to focus on repeal — and party centrists who are sensitive to drops in coverage, while swatting away Democratic critics who say the plan will slash taxes for the rich and benefits for the poor.

Based on early reactions, the release of the analysis didn’t change much.

Sen. Susan Collins, a Republican centrist from Maine, said the estimated drop in coverage is “cause for alarm,” so the House should “slow down and reconsider certain provisions in the bill.”

Rep. Mark Walker, North Carolina Republican and chairman of the conservative Republican Study Committee — a bloc of roughly 170 GOP members — said the score did little to alleviate the group’s concerns.

He said the refundable tax credits still cost too much, while large savings are “built on the shaky assumption that a future Congress will actually maintain the bill’s enrollment freeze in Obamacare’s Medicaid expansion.”

Democrats, meanwhile, say the swaths of the country that voted for Mr. Trump by large margins will be enraged by the changes.

“Today’s CBO report now confirms what we already knew: Despite promises that ‘everyone would be covered’ and ‘no one would be worse off,’ this Republican bill would rip away health insurance from 24 million Americans over the next decade and ask millions to pay more for less coverage,” said Reps. Richard Neal of Massachusetts and Frank Pallone of New Jersey, the ranking Democrats on the Ways and Means committee and the Energy and Commerce committee, respectively.

GOP leaders pushed their legislation through those committees last week without the CBO’s input, enraging Democrats who said Republicans were trying to dash ahead of bad news in the imminent report.

Republicans downplayed the new numbers, saying it’s hard to predict how many people will be covered because their plan tries to drive down costs and provide universal “access” to coverage, rather than mandating everyone to buy it.

Republicans also said the CBO got its projections wrong seven years ago when it tried to predict the number of people who would seek coverage on Obamacare’s exchanges.

Insurers will be able charge older customers up to five times more than younger ones under the GOP plan, rather than the three-to-one ratio under Obamacare, “substantially reducing premiums for young adults and substantially raising premiums for older people,” scorekeepers said.

The GOP bill hopes to entice people to buy coverage through age-based tax credits.

While the effects differ widely depending on individual circumstances, CBO analysts said a 21-year-old making $26,500 would save $250 in premiums in 2026 compared to Obamacare’s subsidies, while a 64-year-old making the same income would pay nearly $13,000 more.

But a 21-year-old making $68,200 would save more than $3,500, and a 64-year-old making that much would save $700.

The CBO said the actuarial value of the plans — the percentage of costs the plan pays for covered services — would be less under Republicans’ bill.

For now, Republicans are forging ahead with a fast-track process that allows them to avoid a filibuster in the Senate.

Democrats have said they will not provide any votes for the repeal plan, though a bevy of conservatives have said they cannot support the plan, either, because the tax credits replace one entitlement with another.

“If it were up today, I have told leadership that I’m a lean no,” Rep. Steve King, Iowa Republican, said before the CBO report’s release. “I’m hopeful that we can fix this, and maybe we have to start over to fix it.”

The plan must also comply with arcane budget rules on the Senate side if it makes it out of the House, posing yet another roadblock to GOP leaders, though the CBO’s score on deficit reduction is a boon for their plans.

Critics of the GOP’s push to redo health care say their plans threaten to upend the individual health market and spark the “death spiral” of rising premiums and dwindling sign-ups that Republican had long warned about under Obamacare.

Yet CBO staff said the market should be relatively stable in the next two years, since Obamacare’s subsidies will still cushion people against rising premiums through 2020, when the GOP plan kicks in.

Also, Republicans have included billions of dollars for a “stability” fund that states can use to help insurers cover high-risk patients and help people pay their out-of-pocket costs.

The CBO also said the individual market would be stable “in most areas” under current law, too — a notable contrast to GOP claims the Obamacare market is totally collapsing.

However, the CBO predicted ripples in the employer-based coverage. Two million fewer people will get insurance through their jobs as of 2020, rising to 7 million fewer by 2026, scorekeepers said.

That’s because large employers will no longer be mandated to offer adequate coverage. Plus, a broader array of workers will qualify for the GOP’s age-based tax credits compared to Obamacare assistance reserved for low and moderate-income people.

However, the CBO said many employers will maintain coverage, since workers are poised to receive less government assistance, on average, under the GOP plan than under Obamacare, and shopping on the individual market can be difficult.